The mechanisms of antigen-specific immune response (Ir) gene control and of antigen-mediated T-lymphocyte activation have been explored with the goal of understanding the regulation of the immune response and learning to manipulate it. We have found two major or immunodominant antigenic sites on myoglobin, our model protein antigen. Each is recognized with a different class II (or Ia) major histocompatibility antigen (I-Ad or I-Ed) so that the presence of one of these Ia molecules leads to selective activation of T-cells specific for one of these sites. Synthetic peptides were made which stimulate T-cell clones specific for these sites and synthetic variants used to define critical amino acid residues. For one site, critical residues were all hydrophilic and on one side of the alpha helical peptide segment, but the hydrophobic side was also necessary to be exposed, as demonstrated by studies of antign processing. Thus, the amphipathic alpha helical structure was important. The other site was also an amphipathic alpha helix. A computer search of the sequences of six proteins with 12 known T-cell antigenic sites revealed that 10 of the 12 sites fell into regions of hydrophobic periodicity compatible with an amphipathic alpha helix, with a chance of random occurrence for each protein of p less than 0.01. This approach may lead us to the biochemical requirements for T-cell recognition of antigen and may be a powerful tool in the search for T-cell sites and the design of synthetic vaccines. Several methods of immunopotentiation were developed. It was found that IL-2 enhanced antibody responses of low responders to the levels of high responders, perhaps by amplifying T-cell help. Also, targeting the antigen to the immune system by coupling it to anti-immunoglobulin led to enhanced uptake and presentation by B cells at low concentration, with a resultant greater than 10-fold increase in potency for stimulation of T cells in vitro and greater than 10-fold increase in immunogenicity for antibody production in vivo. Both of these approachs may allow development of vaccines for weak or scarce antigens, or immunization of immunodeficient patients.